Apparatus with strake elements and methods for installing strake elements

ABSTRACT

There is disclosed an apparatus comprising a plurality of strake elements, each strake element adapted to cover an arc-angle about a circumference of a structural element; and a mechanism adapted to attach the strake elements to each other to cover at least a portion of the circumference of the structural element.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims the benefit of the filing date of U.S.Provisional patent application Ser. No. 60/684,034, filed on May 24,2005, the disclosure of which is incorporated herein by reference.

FIELD OF INVENTION

There is disclosed an apparatus with strake elements and methods forinstalling strake elements.

BACKGROUND

Production of oil and gas from offshore fields has created many uniqueengineering challenges. One of these challenges is dealing with effectsof currents on marine elements. Such marine elements may be employed ina variety of applications, including, e.g., subsea pipelines; drilling,production, import and export risers; tendons for tension leg platforms;legs for traditional fixed and for compliant platforms; other mooringelements for deepwater platforms; and, the hull structure for spar typestructures. These currents may cause vortexes to shed from the sides ofthe marine elements, inducing vibrations that can lead to the failure ofthe marine elements or their supports.

Deepwater production risers, drilling risers, platform export risers,import risers bringing in production from satellite wells, tendons fortension leg platforms, and other conduits for produced fluids anddeepwater mooring elements formed from tubular goods may be typical ofapplications that may have vibration problems. Subsea pipelinestraversing valleys on the ocean floor for extended, unsupported lengthsand spar hulls moored at the end of long tethers and/or mooring linesprovide additional examples.

When these types of structures, such as a cylinder, experience a currentin a flowing fluid environment, it is possible for the structure toexperience vortex-induced vibrations (VIV). These vibrations may becaused by oscillating dynamic forces on the surface which can causesubstantial vibrations of the structure, especially if the forcingfrequency is at or near a structural natural frequency. The vibrationsmay be larger in the transverse (to flow) direction; however, in-linevibrations can also cause stresses which may be sometimes larger thanthose in the transverse direction.

Drilling for and/or producing hydrocarbons or the like from subterraneandeposits which exist under a body of water exposes underwater drillingand production equipment to water currents and the possibility of VIV.

Risers are discussed in this patent document as a non-exclusive exampleof an aquatic structure subject to VIV. A riser system may be used forestablishing fluid communication between the surface and the bottom of awater body. The principal purpose of the riser is to provide a fluidflow path between a drilling vessel and a well bore and to guide a drillstring to the well bore.

A typical riser system normally consists of one or more fluid-conductingconduits which extend from the surface to a structure (e.g., wellhead)on the bottom of a water body. For example, in the drilling of asubmerged well, a drilling riser usually consists of a main conduitthrough which the drill string is lowered and through which the drillingmud is circulated from the lower end of the drill string back to thesurface. In addition to the main conduit, it is conventional to provideauxiliary conduits, e.g., choke and kill lines, etc., which extendparallel to and may be carried by the main conduit.

The magnitude of the stresses on the riser pipe, tendons or spars isgenerally a function of and increases with the velocity of the watercurrent passing these structures and the length of the structure.

There are generally two kinds of current-induced stresses in flowingfluid environments. The first kind of stress is caused by vortex-inducedalternating forces that vibrate the structure (“vortex-inducedvibrations”) in a direction perpendicular to the direction of thecurrent. When fluid flows past the structure, vortices may bealternately shed from each side of the structure. This produces afluctuating force on the structure transverse to the current. If thefrequency of this harmonic load is near the resonant frequency of thestructure, large vibrations transverse to the current can occur. Thesevibrations can, depending on the stiffness and the strength of thestructure and any welds, lead to unacceptably short fatigue lives. Infact, stresses caused by high current conditions in marine environmentshave been known to cause structures such as risers to break apart andfall to the ocean floor.

The second type of stress is caused by drag forces which push thestructure in the direction of the current due to the structure'sresistance to fluid flow. The drag forces may be amplified by vortexinduced vibrations of the structure. For instance, a riser pipe that isvibrating due to vortex shedding will disrupt the flow of water aroundit more than a stationary riser. This may result in more energy transferfrom the current to the riser, and hence more drag.

Some devices used to reduce vibrations caused by vortex shedding fromsubsea structures operate by modifying the boundary layer of the flowaround the structure to prevent the correlation of vortex shedding alongthe length of the structure. Examples of such devices includesleeve-like devices such as helical strake elements, shrouds, fairingsand substantially cylindrical sleeves. Currently available strakeelements and fairings cover an entire circumference of a cylindricalelement or may be clamshell shaped to be installed about thecircumference.

Some VIV and drag reduction devices can be installed on risers andsimilar structures before those structures may be deployed underwater.Alternatively, VIV and drag reduction devices can be installed onstructures after those structures may be deployed underwater.

Elongated structures in wind in the atmosphere can also encounter VIVand drag, comparable to that encountered in aquatic environments.Likewise, elongated structures with excessive VIV and drag forces thatextend far above the ground can be difficult, expensive and dangerous toinstall VIV and/or drag reduction devices.

U.S. Pat. No. 6,561,734 discloses a partial helical strake system andmethod for suppressing vortex-induced-vibration of a substantiallycylindrical marine element, the strake system having a base connected tothe cylindrical marine element and an array of helical strake elementsprojecting from the base for about half or less of the circumference ofthe cylindrical marine element. U.S. Pat. No. 6,561,734 is hereinincorporated by reference in its entirety.

There is a need in the art for an improved apparatus and method forsuppressing vibration.

There is another need in the art for apparatus and methods forsuppressing vibration which do not suffer from the disadvantages of theprior art.

There is another need in the art of apparatus for and new and improvedmethods of manufacturing and installing strake elements for suppressingvibration in a flowing fluid environment.

These and other needs of the present disclosure will become apparent tothose of skill in the art upon review of this specification, includingits drawings and claims.

SUMMARY OF THE INVENTION

One aspect of the disclosed invention provides an apparatus comprising aplurality of strake elements, each strake element adapted to cover anarc-angle about a circumference of a structural element; and a mechanismadapted to attach the strake elements to each other to cover at least aportion of the circumference of the structural element.

Another aspect of the disclosed invention provides a method ofinstalling strake elements comprising providing a plurality of strakeelements, each comprising an arc angle from 30° to 180°; and connectingeach strake element to at least one other strake element about acircumference of a structural element.

Improvements and advantages of the invention include one or more of thefollowing: an improved strake element manufacturing system and method,an improved strake element installing system and method, a moreefficient strake element installing system and method, and/or animproved system and method for installing strake elements about existingstructural elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an offshore system.

FIG. 2 a is a cross-sectional view of a tubular with strake elements.

FIG. 2 b is a side view of a tubular with strake elements.

FIG. 2 c is an end view of a tubular with strake elements.

FIG. 3 a is a side view of a tubular with strake elements.

FIG. 3 b is an end view of a tubular with strake elements.

FIG. 4 a is an end view of a tubular with strake elements.

FIG. 4 b is a side view of a tubular with strake elements.

FIG. 4 c is a side view of a tubular with strake elements.

FIG. 5 a is an end view of a tubular with strake elements.

FIG. 5 b is a side view of a tubular with strake elements.

FIG. 5 c is an end view of a tubular with strake elements.

FIG. 5 d is an end view of a tubular with strake elements.

FIG. 6 a is an end view of a strake element.

FIG. 6 b is an end view of a tubular with strake elements.

FIG. 6 c is a side view of a tubular with strake elements.

FIG. 7 a is an end view of a shell to form strake elements.

FIG. 7 b is a side view of a shell to form strake elements.

FIG. 7 c is a side view of a shell to form strake elements.

FIG. 7 d is an end view of a shell being used to form strake elements.

FIG. 7 e is an end view of a shell being used to form strake elements.

FIG. 8 a is an end view of a die to form strake elements.

FIG. 8 b is an end view of a die to form strake elements.

FIG. 8 c is a side view of an extruder to form strake elements.

DETAILED DESCRIPTION OF THE INVENTION

In one embodiment there is disclosed an apparatus comprising a pluralityof strake elements, each strake element adapted to cover an arc-angleabout a circumference of a structural element; and a mechanism adaptedto attach the strake elements to each other to cover at least a portionof the circumference of the structural element. In some embodiments, thestructural element comprises a structure selected from the groupconsisting of a tubular, a pipe, a rod, a buoyancy device, a riser, anda mooring line. In some embodiments, the mechanism for attachingcomprises a plurality of bolts and nuts. In some embodiments, themechanism for attaching the strake elements comprises a plurality ofheat welds. In some embodiments, the apparatus also includes 4 helicalstrake elements, each strake element comprising an arc-angle from 70° to120°, for example from 80° to 100°. In some embodiments, each strakeelement overlaps the next strake element, to form a plurality of strakeshapes about the circumference of the cylindrical element, the strakeshapes selected from the group consisting of triangles, rectangles, andtrapezoids. In some embodiments, the cylindrical element comprises anoutside diameter from 5 to 60 cm and/or wherein the strake elements havea height from 5% to 50% of an outside diameter of the structural elementand/or wherein the strake elements are helical. In some embodiments, theapparatus also includes the structural element about which the pluralityof strake elements has been attached. In some embodiments, the apparatusalso includes from 2 to 8 strake elements. In some embodiments, theapparatus also includes a pitch between each strake element from 0.1 to10 meters.

In one embodiment there is disclosed a method of installing strakeelements comprising providing a plurality of strake elements, eachcomprising an arc angle from 30° to 180°; and connecting each strakeelement to at least one other strake element about a circumference of astructural element. In some embodiments, at least one of the strakeelements has been produced by extruding the element through a die. Insome embodiments, at least one of the strake elements has been producedby forming the element in a mold. In some embodiments, at least one ofthe strake elements has been produced by placing a moldable sheet on astrake shape, forming the moldable sheet about the desired strake shape,and removing the moldable sheet from the shape. In some embodiments, themethod also includes applying suction through the strake shape, to forcethe moldable sheet about the desired shape. In some embodiments,connecting each strake element to at least one other strake elementcomprises bolting the elements to each other. In some embodiments,connecting each strake element to at least one other strake elementcomprises welding the elements to each other. In some embodiments,connecting each strake element to at least one other strake elementcomprises overlapping adjacent elements to form a desired strake shape.In some embodiments, the structural element comprises a structureselected from the group consisting of a tubular, a pipe, a rod, abuoyancy device, a riser, and a mooring line.

Referring first to FIG. 1, there is illustrated offshore system 100,with which the invention may be used. System 100 includes floatingplatform, 110 with facilities 105 on top. Platform is floating in a bodyof water having water surface 115 and bottom of the body of water 135.Buoyancy device 120 keeps platform 110 from sinking. Riser 125 connectsplatform 110 with well 140. Mooring lines 130 anchor platform 110 to thebottom of the body of water 135.

Vortex induced vibration (VIV) may cause vibration of a structuralelement, such as one or more of buoyancy device 120, riser 125, and/ormooring lines 130. In some embodiments of the invention, one or morestrake elements and/or fairings may be applied to one of more ofbuoyancy device 120, riser 125, and/or mooring lines 130. Suitablestructural elements may include tubulars, pipes, rods, buoyancy device120, riser 125, and/or mooring lines 130. Vortex induced vibration (VIV)may also cause vibration of other subsea structural elements to whichthe invention may be applied.

Referring now to FIGS. 2 a-2 c, in some embodiments of the invention,structural element 204 is illustrated. Structural element 204 enclosespassage 202. Strake elements 206 a, 206 b, 206 c, and 206 d may bemounted about the circumference of structural element 204. Strakeelements 206 a-206 d serve to inhibit vibration when structural element204 is in a flowing fluid stream.

Structural element 204 has outside diameter D 218. Strake elements 206a-206 d have a height H 220. Adjacent strake elements may be spacedapart by an arc-angle 222. In some embodiments of the invention, outsidediameter D 218 may be from about 2 to 60 cm. In some embodiments of theinvention, height H 220 may be from about 5% to about 50% of outsidediameter D 218. In some embodiments of the invention, height H 220 maybe from about 1 to about 15 cm. In some embodiments of the invention,arc-angle 222 may be from about 30 to about 180 degrees. In someembodiments of the invention, arc-angle L 222 may be from about 60 toabout 90 degrees.

Referring now to FIGS. 3 a-3 b, in some embodiments of the invention,two helical-shape strake elements 306 a and 306 b may be installed aboutstructural element 304. Structural element 304 encloses fluid passage302. Strake elements 306 a and 306 b extend outside the circumference ofstructural element 304. Strake elements 306 a and 306 b have rectangularstrake shapes that may be mounted at an angle relative to a longitudinalaxis 305 of structural element 304. An end view of structural element304 and strake elements 306 may be seen in FIG. 3 b showing rectangularstrake shapes.

Structural element 304 has outside diameter D 328. Strake elements 306 aand 306 b have height H 330. Adjacent strake elements 306 a-306 d may bespaced apart by pitch L 332, and there may be two strake starts: strakeelements 306 a and 306 b.

In some embodiments of the invention, outside diameter D 328 may be fromabout 2 to about 100 cm. In some embodiments of the invention, height H330 may be from about 5% to about 50% of outside diameter D 328. In someembodiments of the invention, height H 330 may be from about 1 to about20 cm. In some embodiments of the invention, pitch L 332 may be fromabout 1D to about 10D. In some embodiments of the invention, pitch L 332may be from about 10 to about 500 cm.

In some embodiments of the invention, the number of strake starts may befrom about 2 to about 10, for example from about 4 to about 6.

Referring now to FIGS. 4 a-4 b, in some embodiments of the invention, a4-start strake system may be installed about structural element 404.Structural element 404 encloses passage 402. Strake elements 406 a, 406b, 406 c, and 406 d may be spaced about the circumference of structuralelement 404. In some embodiments of the invention, strake elements 406a-406 d may be rigidly connected to first support 410 at one end, andsecond support 412 at the other end, as shown in FIG. 4 b. Strakeelements 406 a-406 d may be helical strake elements mounted aboutstructural element 404. Supports 410 and 412 may be locked relative toeach other, for example by locking mechanism 414, as shown in FIG. 4 b.

Structural element 404 has outside diameter D 458. Strake elements 406a-406 d have height H 460. Strake elements 406 a-406 d may be spacedapart by pitch L 462.

In some embodiments of the invention, outside diameter D 458 may be fromabout 3 to about 50 cm. In some embodiments of the invention, height H460 may be from about 5% to about 50% of outside diameter D 458. In someembodiments of the invention, height H 460 may be from about 2 to about10 cm. In some embodiments of the invention, pitch L 462 may be fromabout 1D to about 10D. In some embodiments of the invention, pitch L 462may be from about 10 to about 100 cm.

Strake elements 406 a-406 d have a rectangular strake shape forming anangle relative to axis 450.

In some embodiments of the invention, referring to FIG. 4 c, strakeelements 406 a-406 d may be mounted about structural element 404. Bands422, 424, 426, 428, and 430 may be placed about the circumference ofstructural element 404, to lock strake elements 406 a-406 d in place.

Strake elements 406 a-406 d form an angle with axis 450 of structuralelement 404.

In some embodiments of the invention, there may be about 2 to about 10helical strake starts about a circumference of structural element 404.In some embodiments of the invention, there may be about 3 to about 6helical strake starts about a circumference of structural element 404.In some embodiments of the invention, there may be about 4 helicalstrake starts about a circumference of structural element 404.

Referring now to FIGS. 5 a-5 b, in some embodiments of the invention,strake element system 500 is illustrated. System 500 includes structuralelement 504 enclosing passage 502. Strake elements 506 a, 506 b, 506 c,and 506 d may be placed about structural element 504, and connected toeach other. In some embodiments of the invention, elements 506 a-506 dmay be formed into the desired shapes, and then placed about structuralelement 504, for example by bending a sheet, molding the elements,vacuum forming the elements, and/or extruding the elements. Each ofstrake elements 506 a, 506 b, 506 c, and 506 d covers an arc-angle abouta circumference of structural element 504 of about 90°.

In some embodiments, there may be provided about two strake elementseach covering an arc-angle about a circumference of structural element504 of about 180°. In some embodiments, there may be provided aboutthree strake elements each covering an arc-angle about a circumferenceof structural element 504 of about 120°. In some embodiments, there maybe provided about five strake elements each covering an arc-angle abouta circumference of structural element 504 of about 72°. In someembodiments, there may be provided about six strake elements eachcovering an arc-angle about a circumference of structural element 504 ofabout 60°. In some embodiments, there may be provided from about two toabout 12 strake elements each covering an arc-angle about acircumference of structural element 504 from about 30° to about 180°.

Structural element 504 has outside diameter D 558. Strake elements 506a-506 d have height 560. Adjacent strake elements 506 a-506 d may bespaced apart by pitch L 562.

In some embodiments of the invention, outside diameter D 558 may be fromabout 2.5 to 50 cm. In some embodiments of the invention, height H 560may be from about 5% to about 50% of outside diameter D 558. In someembodiments of the invention, height H 560 may be from about 1.25 to 15cm. In some embodiments of the invention, pitch L 562 may be from about1D to about 10D. In some embodiments of the invention, pitch L 562 maybe from about 15 cm to 60 cm.

Referring now to FIG. 5 c, in some embodiments of the invention, amethod of connecting elements 506 a-506 d together is illustrated. Bolt510 a and nut 510 b may be used to connect element 506 a and element 506b at the strake portion of those elements. Bolt 512 a and nut 512 b maybe used to connect elements 506 b and 506 c. Bolt 514 a and nut 514 bmay be used to connect elements 506 c and element 506 d. Bolt 516 a andnut 516 b may be used to connect element 506 d and element 506 a.

In some embodiments of the invention, referring to FIG. 5 d, a method ofconnecting elements 506 a-506 d together is illustrated. Heat weld 520is used to secure a portion of element 506 a and 506 b at the strakeportion where the two elements meet. Heat weld 522 has been used toconnect element 506 b and 506 c. Heat weld 524 has been used to connectelement 506 c and element 506 d. Heat weld 526 has been used to connectelement 506 d and 506 a.

In some embodiments of the invention, other methods of connectingelements 506 a-506 d may be used, for example, rivets, male and femaleconnections such as a pin and a groove, an end of one element fittinginto a slot on an adjacent element, or other mechanical connections asare known in the art.

Referring now to FIGS. 6 a-6 c, in some embodiments of the invention,strake element system 600 is illustrated. System 600 includes structuralelement 604 enclosing passage 602. Individual strake elements 606 may beinter-connected, as shown in FIG. 6 b, so that element 606 a overlapselement 606 b, which overlaps element 606 c, which overlaps element 606d, which overlaps element 606 a. In some embodiments of the invention,elements 606 a-606 d may be identical. In some embodiments, elements 606a-606 d overlap to form a trapezoidal strake shape.

Structural element 604 has outside diameter D 658. Strake elements 606a-606 d have height H 660. Adjacent strake elements 606 a-606 d may bespaced apart by pitch L 662.

In some embodiments of the invention, outside diameter D 658 may be fromabout 3 to about 50 cm. In some embodiments of the invention, height H660 may be from about 5% to about 50% of outside diameter D 658. In someembodiments of the invention, height H 660 may be from about 5 to 15 cm.In some embodiments of the invention, pitch L 662 may be from about 1Dto about 10D. In some embodiments of the invention, pitch L 662 may befrom about 10 cm to 50 cm.

In some embodiments of the invention, four elements 606 a-606 d eachencompassing a 90° arc-angle may be used about the circumference ofstructural element 604. In some embodiment, three elements, eachencompassing a 120° arc-angle about circumference of structural element604 may be used. In some embodiments of the invention, about 2, 5, 6, ormore elements may be used to form strake elements about structuralelement 604.

In some embodiments of the invention, elements 606 when interconnectedform a helical structure about structural element 604, as shown in FIG.6 c.

In some embodiments of the invention, elements 606 form a strake shapehaving an angle with axis 650.

Referring now to FIGS. 7 a-7 e, in some embodiments of the invention, amethod and apparatus for forming strake elements will be illustrated.Strake element manufacturing system 700 includes shell 702 enclosingchamber 704 with holes 706 defined within shell 702 into chamber 704.Shell 702 defines strake shape 708. In some embodiments of theinvention, as illustrated in FIG. 7 b, shell 702 may be used tomanufacture a strake element with a long pitch and/or a large number ofstarts. In some embodiments of the invention, referring to FIG. 7 c,shell 702 may be used to manufacture a strake element with a short pitchand/or a small number of starts.

In operation, referring to FIGS. 7 d-7 e, moldable sheet 720 is placedabove shell 702. Sheet 720 is heated or otherwise processed so that itis moldable, and then placed on top of shell 702. Suction is applied tochamber 704, which applies suction to holes 706, to force sheet 720about shell 702 to take on desired strake shape 708. After sheet hastaken on the desired shape, it may be cooled or otherwise processed andthen removed from shell 702, and another sheet placed over shell 702.

In some embodiments of the invention, sheet 720 may be made of apolymer, such as a thermoplastic polymer or a thermosetting polymer, forexample polypropylene, polyethylene, other polyolefins, or co-polymersof olefins.

In some embodiments of the invention, the strake elements may be made insections of varied length, based on oven capabilities at the moldingfacility. The strake elements may be manufactured in halves, or could bemanufactured in thirds or more, for example quadrants. The moldingprocess described is not limited to a specific number of starts.

In some embodiments of the invention, the manufacturing method usesstate-of-the-art vacuum forming techniques. A pattern shell 702 may bebuilt which represents the inside surface of the helical strake piecedesired. The pattern shell 702 can be constructed from a wide array ofmaterials.

In some embodiments of the invention, for small quantity runs, thepattern shell 702 used for the vacuum forming can be constructed from amaterial such as fiberglass. A fiberglass mold is laid up over thepattern, with a parting flange, which allows the fiberglass mold to beseparated from the pattern. The fiberglass mold may be then dressed andused to lay up fiberglass tooling shell 702 which will have appropriateholes 706 and vacuum access attached. The tooling shell 702 can then beused to form helical strake sections from any of a variety ofthermoplastic compounds, for example polypropylene, polyethylene, orcopolymers of olefins.

In some embodiments of the invention, for large production runs, thepattern can be used to make a sturdy tooling shell 702 from aluminum orother materials using sand casting or other state-of-the-artmanufacturing techniques.

Referring now to FIGS. 8 a-8 c, in some embodiments of the invention, amethod and apparatus for forming strake elements will be illustrated.Strake element manufacturing system 800 includes die 806 a through whicha strake element 814 may be extruded. One suitable strake shape 808 a isshown in die 806 a, which may be used to form helical strake elements814, which may be connected to each other and installed about astructural element. Another suitable strake shape 808 b is shown in die806 b, which may be used to form helical strake elements 814, which maybe connected to each other and installed about a structural element.System 800 also includes extruder 810, for example a screw extruder or apiston which may be used to melt and pressurize a polymer or othermeltable material such as metal, then force it through die 806. Hopper812 may be used to store the extrudable material prior to beingintroduced into extruder 810.

In operation, a material may be introduced into hopper 812. Gravityand/or extruder 810 then pull material into extruder 810. Material maybe melted and forced through die 806 to form desired strake shape 814. Acooling means, for example a cooling bath or a fan may be provideddownstream of the die 806 to cool the strake 814.

In some embodiments of the invention, strake sections may be applied toone side of a riser in a manner that all of the strake sections may bemounted to the riser away from the rollers. After the riser, with theattached strake sections, passes through the rollers, the strakesections may be spaced equidistant around the riser with the correctspacing and pitch, for example manually or by an ROV.

In some embodiments of the invention, strake sections may be attached tothe riser with an interference fit, which may eliminate the need to usecollars. The flanges of the strake sections could be shaped in an “Aconfiguration” rather than flat to allow some flange flexibility toprovide an interference fit to the riser. The flanges would be fastenedusing snaps or flange wraps.

In some embodiments of the invention, a hinged upper collar and a hingedlower collar may be mounted to cylindrical elements, such thatindividual strake sections would insert into the collars and lock on theriser without the use of bands or other fastening devices. Velcro may beused on the upper flanges of the strake section to temporarily hold thestrake sections to the riser while the upper collar may be installed.The sequence of operations might be to install the lower collar first.Then install the strake sections about a cylindrical element. Lastinstall the upper collar to lock the strake sections in place about thecylindrical element. The collars could be designed such that the lowerportion of the collar would lock in the upper flanged strake sectionsand the upper portion of collar would provide the base to install thenext section of flanged strake sections.

In some embodiments of the invention, there is disclosed a system ofreducing vibration, including a structural element, a plurality ofstrake elements spaced about a circumference of the cylindrical element,wherein the cylindrical element comprises a longitudinal axis, andwherein the strake elements may be substantially aligned with the axis,and wherein the strake elements may be attached at a first end to afirst support, and wherein the strake elements may be attached at asecond end to a second support, wherein the first support and the secondsupport may be adapted to be rotated relative to one another, so thatthe strake elements form a helix about the cylindrical element. In someembodiments of the invention, the number of strake elements may be about2 to about 6 strake elements. In some embodiments of the invention, thenumber of strake elements may be about 4 strake elements. In someembodiments of the invention, the system also includes a locking deviceto rotationally lock the first support and the second support relativeto one another. In some embodiments of the invention, the stakes have aheight of about 5% to about 50% of an outside diameter of thecylindrical element. In some embodiments of the invention, the strakeelements have a height of about 0.5 to about 15 cm. In some embodimentsof the invention, the strake elements may be spaced apart by a length ofabout 15 to about 150 cm. In some embodiments of the invention, thestrake elements may be spaced apart by length of about 1 to about 20outside diameters of the cylindrical element.

In some embodiments of the invention, there is disclosed a method ofreducing vibration in a cylindrical element, including providing astructural element, placing a plurality of strake elements about acircumference of the cylindrical element, substantially in alignmentwith a longitudinal axis of the cylindrical element, attaching a firstend of the strake elements to a first support, attaching a second end ofthe strake elements to a second support, and rotating the first supportrelative to the second support, to impart a helical twist to the strakeelements. In some embodiments of the invention, the method also includeslocking the first support relative to the second support. In someembodiments of the invention, the number of strake elements may be fromabout 2 to about 6. In some embodiments of the invention, the number ofstrake elements may be about 4.

In some embodiments of the invention, there is disclosed a system forreducing vibration, including a cylindrical element, a plurality ofstrake elements about a circumference of the cylindrical element, thestrake elements substantially aligned with a longitudinal axis of thecylindrical element, and a device adapted to receive the strake elementsand impart a desired pitch to the strake elements about thecircumference of the cylindrical element. In some embodiments of theinvention, the system also includes a plurality of locking devicesadapted to secure the strake elements at the desired pitch.

In some embodiments of the invention, there is disclosed a method ofminimizing vibration, including providing a cylindrical element, placinga plurality of strake elements substantially aligned with a longitudinalaxis of a cylindrical element, the strake elements spaced about acircumference of the cylindrical element, sliding a device over thestrake elements to impart a desired pitch to the strake elements. Insome embodiments of the invention, the method also includes locking thestrake elements in place at the desired pitch. In some embodiments ofthe invention, the number of strake elements may be from about 2 toabout 6, for example about 4. In some embodiments of the invention, thedesired pitch comprises an angle of about 15 to about 75°. In someembodiments of the invention, the desired pitch comprises an angle ofabout 30 to about 60°.

In some embodiments of the invention, there is disclosed an apparatus,including a structural element, a plurality of strake elements, eachcovering an arc segment about a circumference of the cylindricalelement, and a mechanism for attaching the strake elements to each otherto cover the entire outer circumference of the cylindrical element. Insome embodiments of the invention, the mechanism for attaching comprisesa plurality of bolts and nuts. In some embodiments of the invention, themechanism for attaching the strake elements comprises a plurality ofheat welds. In some embodiments of the invention, the apparatus includes4 strake elements, each strake element comprising an arc angle of about90°. In some embodiments of the invention, each strake element overlapsthe next strake element, to form a plurality of strake shapes about thecircumference of the cylindrical element.

In some embodiments of the invention, there is disclosed a system formanufacturing strake elements, including a shell, the shell defining aninterior chamber and an exterior surface, the exterior surface defininga desired strake shape, the exterior surface comprising a plurality ofholes defined therethrough, wherein the system may be adapted to receivea moldable sheet which can be forced about the desired strake shape.

In some embodiments of the invention, there is disclosed a method ofmanufacturing a strake element, including placing a moldable sheet on astrake shape, heating the moldable sheet to form about the desiredstrake shape, cooling the moldable sheet, and removing the moldablesheet from the shape. In some embodiments of the invention, the methodalso includes applying suction through the strake shape, to force themoldable sheet about the desired shape. In some embodiments of theinvention, the strake shape defines a helix in a longitudinal direction.

In some embodiments of the invention, clamshell type strake elements maybe mounted around a pipe according to the method disclosed in U.S. Pat.No. 6,695,539, which is herein incorporated by reference in itsentirety.

In some embodiments of the invention, strake elements may be installedabout a pipe according to the method disclosed in U.S. Pat. No.6,561,734, which is herein incorporated by reference in its entirety.

In some embodiments of the invention, strake elements may be installedabout a pipe according to the method disclosed in United States PatentApplication Publication No. 2003/0213113, which is herein incorporatedby reference in its entirety.

In some embodiments of the invention, the outside diameter of a pipe towhich strake elements can be attached may be from about 10 to about 50cm. In some embodiments of the invention, the height of strake elementsmay be from about 5% to about 50% of the pipe's outside diameter. Insome embodiments of the invention, the height of strake elements may befrom about 5 to about 20 cm.

In some embodiments of the invention, arc-angle between adjacent strakeelements may be from about 30 to about 180 degrees. In some embodimentsof the invention, arc-angle between adjacent strake elements may be fromabout 60 to about 90 degrees.

In some embodiments of the invention, the structural element may becylindrical, or have an elliptical, oval, or polygonal cross-section,for example a square, pentagon, hexagon, or octagon.

In some embodiments of the invention, suitable risers 125, buoyancydevices 120, and/or mooring lines 130 have an outside diameter of about5 to 100 cm. In some embodiments of the invention, suitable risers 125,buoyancy devices 120, and/or mooring lines 130 have a outside diameterof about 10 to 50 cm. In some embodiments of the invention, suitablerisers 125, buoyancy devices 120, and/or mooring lines 130 have aoutside diameter of about 20 to 30 cm.

In some embodiments of the invention, suitable risers 125, buoyancydevices 120, and/or mooring lines 130 have a wall thickness of about 0.1to 5 cm. In some embodiments of the invention, suitable risers 125,buoyancy devices 120, and/or mooring lines 130 have a wall thickness ofabout 0.2 to 3 cm. In some embodiments of the invention, suitable risers125, buoyancy devices 120, and/or mooring lines 130 have has a wallthickness of about 0.5 to 2 cm.

In some embodiments of the invention, suitable risers 125, buoyancydevices 120, and/or mooring lines 130 may be made of a carbon steelpipe.

Those of skill in the art will appreciate that many modifications andvariations are possible in terms of the disclosed embodiments,configurations, materials and methods without departing from theirspirit and scope. Accordingly, the scope of the claims appendedhereafter and their functional equivalents should not be limited byparticular embodiments described and illustrated herein, as these aremerely exemplary in nature.

1. An apparatus, comprising: a plurality of strake elements, each strakeelement adapted to cover an arc-angle about a circumference of astructural element; and a mechanism adapted to attach the strakeelements to each other to cover at least a portion of the circumferenceof the structural element.
 2. The apparatus of claim 1, wherein thestructural element comprises a structure selected from the groupconsisting of a tubular, a pipe, a rod, a buoyancy device, a riser, anda mooring line.
 3. The apparatus of claim 1, wherein the mechanism forattaching comprises a plurality of bolts and nuts.
 4. The apparatus ofclaim 1, wherein the mechanism for attaching the strake elementscomprises a plurality of heat welds.
 5. The apparatus of claim 1,comprising 4 helical strake elements, each strake element comprising anarc-angle from 70° to 120°.
 6. The apparatus of claim 1, wherein eachstrake element overlaps the next strake element, to form a plurality ofstrake shapes about the circumference of the cylindrical element, thestrake shapes selected from the group consisting of triangles,rectangles, and trapezoids.
 7. The apparatus of claim 1, wherein thecylindrical element comprises an outside diameter from 5 to 60 cm,wherein the strake elements have a height from 5% to 50% of an outsidediameter of the structural element, and wherein the strake elements arehelical.
 8. The apparatus of claim 1, further comprising the structuralelement about which the plurality of strake elements has been attached.9. The apparatus of claim 1, comprising from 2 to 8 strake elements. 10.The apparatus of claim 1, comprising a pitch between each strake elementfrom 0.1 to 10 meters.
 11. A method of installing strake elementscomprising: providing a plurality of strake elements, each comprising anarc angle from 30° to 180°; and connecting each strake element to atleast one other strake element about a circumference of a structuralelement.
 12. The method of claim 11, wherein at least one of the strakeelements has been produced by extruding the element through a die. 13.The method of claim 11, wherein at least one of the strake elements hasbeen produced by forming the element in a mold.
 14. The method of claim11, wherein at least one of the strake elements has been produced byplacing a moldable sheet on a strake shape, forming the moldable sheetabout the desired strake shape, and removing the moldable sheet from theshape.
 15. The method of claim 14, further comprising applying suctionthrough the strake shape, to force the moldable sheet about the desiredshape.
 16. The method of claim 11, wherein connecting each strakeelement to at least one other strake element comprises bolting theelements to each other.
 17. The method of claim 11, wherein connectingeach strake element to at least one other strake element compriseswelding the elements to each other.
 18. The method of claim 11, whereinconnecting each strake element to at least one other strake elementcomprises overlapping adjacent elements to form a desired strake shape.19. The method of claim 11, wherein the structural element comprises astructure selected from the group consisting of a tubular, a pipe, arod, a buoyancy device, a riser, and a mooring line.